An on-demand type inkjet printer employs a piezoelectric ink jet head that comprises a pressure chamber 2 filled with an ink, a nozzle 3 that communicates with the pressure chamber 2 and has ink meniscus formed inside thereof from the ink that fills the pressure chamber 2, a piezoelectric element 9 that is deformed when a drive voltage is applied, and an oscillator plate 7 that is stacked on the piezoelectric element 9 so as to form a drive section D, as shown in FIGS. 2 and 3.
In the piezoelectric ink jet head, the drive section D transmits a force generated by the piezoelectric element 9 as a pressure to the ink contained in the pressure chamber 2 thereby to function as a drive power source that discharges ink droplets through the nozzle 3 that communicates with the pressure chamber 2. That is, in the drive section D, as the piezoelectric element 9 deforms due to a drive voltage applied thereto, the oscillator plate 7 is caused to deflect so as to protrude toward the pressure chamber 2 as indicated by dot and dash line in FIG. 2, thereby decreasing the volume of the pressure chamber 2 and pressurizing the ink in the pressure chamber 2, so that ink droplet is discharged from the tip of the nozzle 3.
At the same time, since the oscillator plate 7 is also caused by the pressure of the ink contained in the pressure chamber 2 to deflect in a direction opposite to that shown in the drawing, the drive section D also acts as an elastic body with respect to the vibration of the ink in the head.
When a voltage is applied to the piezoelectric element 9 so as to generate a force, the ink contained in the head experiences vibration under the pressure transmitted via the oscillator plate 7 from the drive section D. This vibration is generated with the drive section D and the pressure chamber 2 acting as the elasticity against the inertia of a feeder port 5 for feeding the ink to the pressure chamber 2, a nozzle passage 4 that communicates the pressure chamber 2 and the nozzle 3, and the nozzle 3. Natural period of vibration of the ink contained in the head during this vibration is determined by the dimensions of the components described above, physical properties of the ink and dimensions and physical properties of the drive section D.
In the piezoelectric ink jet head, an ink droplet is discharged by utilizing the vibration of ink meniscus in the nozzle 3 due to the vibration of the ink described above.
As described in Japanese Unexamined Patent Publication JP-H02-192947-A2 (1990), the piezoelectric ink jet head generally employs such a drive method as described below. A constant drive voltage is continuously applied to a piezoelectric element in the state of standby so that the piezoelectric element is kept deformed and the oscillator plate continue to deflect, thereby to maintain the pressure chamber in a state of decreased volume. To form a dot,    (1) the drive voltage is removed immediately before forming the dot so as to cancel the deformation of the piezoelectric element and cancel the deflection of the oscillator plate, thereby increasing the volume of the pressure chamber with the ink meniscus in the nozzle being pulled toward the pressure chamber, then    (2) the drive voltage is applied again so as to cause the piezoelectric element to deform and the oscillator plate to deflect, thereby decreasing the volume of the pressure chamber and discharge an ink droplet through the tip of the nozzle. This drive method may be referred to as the “Pull-push drive method”in the following description.
FIG. 17 is a simplified graph showing the relation between drive voltage waveform (indicated by a thick dot and dash line) of drive voltage Vp applied to the piezoelectric element and changes in volumetric velocity of ink [indicated by thick solid line with the ink discharging direction indicated by (+)] in the nozzle when the drive voltage waveform is applied with the Pull-push drive method.
This drive method will be described below taking an example in such a case that employs the piezoelectric element 9 of transverse vibration mode formed in a flat plate or layer of small thickness, which contracts in the direction of plane when a drive voltage is applied, as shown in FIGS. 2 and 3.
In the standby period to the left of t1, in FIG. 17, drive voltage Vp is maintained at VH (Vp=VH) so that the piezoelectric element continues to contract in the direction of plane thereby keeping the oscillator plate to deflect in a constant shape so as to maintain the pressure chamber in the state of reduced volume, during which the ink in the head remains stationary, namely the volumetric velocity of ink in the nozzle remains zero.
In order to discharge an ink droplet through the nozzle so as to form a dot on a sheet of paper, the drive voltage Vp applied to the piezoelectric element is removed (Vp=0) at time t1, immediately before the formation so as to cancel the contraction of the piezoelectric element in the direction of plane and cancel the deflection of the oscillator plate.
This results in a predetermined amount of increase in the volume of the pressure chamber, and therefore a quantity of ink in the nozzle corresponding to the volume increase is pulled toward the pressure chamber with ink meniscus drawn thereby. During this step, volumetric velocity of the ink in the nozzle increases in the direction of (−), and then gradually decreases to approach zero as indicated in the period from t1 to t2 in FIG. 17. These changes occur in about a half of the natural period of vibration T1 of the volumetric velocity of ink indicated by thick solid line in the drawing.
At time t2 when the volumetric velocity of ink in the nozzle has approached zero, the drive voltage Vp is applied again to the value of VH (Vp=VH) so that the piezoelectric element contracts in the direction of plane thereby to cause the oscillator plate to deflect. This operation is equivalent to the application of such a drive voltage Vp to the piezoelectric element that has drive voltage waveform of pulse width T3 that is one half of the natural period of vibration T1, as indicated by thick dot and dash line.
This causes a pressure to be exerted by the ink that has been pushed out of the pressure chamber, as the oscillator plate deflects to decrease the volume of the pressure chamber at the time when the ink meniscus is about to return in the direction (+) from the state of being pulled toward the pressure chamber with a maximum displacement (state of zero volumetric velocity at time t2). As a result, the ink protrudes significantly from the tip of the nozzle in the direction of (+). Since the ink protruding from the nozzle tip has an appearance of substantially cylindrical shape, the ink in the protruded state is generally called the ink column. When the ink column has extended to the maximum, a droplet departs from the distal end of the ink column, flies and reaches the paper surface, thereby to form a dot.
The piezoelectric ink jet head generally may employ such a drive method as: a piezoelectric element in the state of standby is maintained in such a condition that drive voltage is not applied thereto, and
when forming a dot,    (I) the drive voltage is applied immediately before forming the dot so as to cause the piezoelectric element to contract and the oscillator plate to deflect, thereby decreasing the volume of the pressure chamber so that the ink meniscus in the nozzle is pushed toward the tip of the nozzle and the ink protrudes from the tip of the nozzle like a column (ink column), then    (II) the drive voltage is removed again so as to cancel the contraction of the piezoelectric element and cancel the deflection of the oscillator plate, thereby increasing the volume of the pressure chamber and pulling back the ink column that has been protruding from the tip of the nozzle into the nozzle, thereby to separate an ink droplet. This drive method may be referred to as the “Push-pull drive method” in the following description.
FIG. 18 is a simplified graph showing the relation between the drive voltage waveform of the drive voltage Vp applied to the piezoelectric element and changes in the volumetric velocity of ink in the nozzle when the drive voltage waveform is applied with the Push-pull drive method.
This drive method will be described below.
In the standby period to the left of t1 in FIG. 18, the drive voltage Vp is not applied (Vp=0) so that the volume of the pressure chamber remains at the initial value, and the volumetric velocity of ink in the nozzle remains zero.
In order to discharge an ink droplet through the nozzle so as to form a dot on a sheet of paper, the drive voltage Vp applied to the piezoelectric element is increased to VH (Vp=VH) at time t1 immediately before the dot formation so as to cause the piezoelectric element to contract in the direction of plane and the oscillator plate to deflect.
This results in a predetermined amount of decrease in the volume of the pressure chamber, and therefore a quantity of ink in the nozzle corresponding to the volume decrease is pushed toward the outside of the nozzle together with ink meniscus. During this step, volumetric velocity of the ink in the nozzle increases in the direction of (+) to reach a maximum, then decreases to approach zero,then increases in the direction of (−) to reach a maximum, and then decreases to approach zero as indicated in the period from t1 to t2 in FIG. 18. These changes occur in the natural period of vibration T1 of the volumetric velocity of ink indicated by a thick solid line in the drawing.
Movement of the ink during the step described above is as follows. First, the ink in the nozzle is pushed toward the outside of the nozzle by the first deflection of the oscillator plate. Then as the volumetric velocity of the ink in the nozzle increases in the direction of (−) due to the intrinsic vibration of the ink, a force to pull the ink back into the nozzle acts on the ink that has been pushed toward the outside of the nozzle. However, since front of the ink that has been pushed toward the outside of the nozzle continues to move toward the outside, the ink is prolonged from the ink meniscus toward the outside, so that the ink column is formed.
At time t2 when the volumetric velocity of ink in the nozzle has passed the point of zero, the drive voltage Vp is removed again (Vp=0) so as to cancel the contraction of the piezoelectric element in the direction of plane thereby to cancel the deflection of the oscillator plate. This operation is equivalent to the application of such a drive voltage Vp to the piezoelectric element that has a drive voltage waveform of pulse width T3 which is proximate to the natural period of vibration T1, as indicated by a thick dot and dash line.
While the ink meniscus in the nozzle is at the deepest position retracted toward the pressure chamber at the time when the volumetric velocity of the ink in the nozzle is zero, it is then urged to move again toward the outside of the nozzle by the intrinsic vibration of the ink. That is, at time t2, the ink meniscus in the nozzle is in the course of moving from the deepest position retracted toward the pressure chamber toward the outside of the nozzle.
Consequently, if the ink is vibrated with reverse phase by canceling the deflection of the oscillator plate and increasing the volume of the pressure chamber at time t2, the movement of the ink meniscus described above is suppressed so that the ink column is separated and an ink droplet is formed. As the ink droplet reaches the paper surface, a dot is formed on the paper.
In the piezoelectric ink jet head driven by the Pull-push or the Push-pull drive method described above, the drive section comprising the piezoelectric element and the oscillator plate vibrates at a natural frequency thereof. Period of the vibration is as small as a few tenths to a fifth of pulse width T3 of the drive voltage waveform.
Making a description by way of the Pull-push drive method, the natural vibration is superposed as ensuing vibration over the vibration of the volumetric velocity of the ink during the formation of ink droplet as shown in FIG. 19. This results in the problem of fluctuations in the volume and flying speed of the ink droplet due to a deviation between the timing of drive voltage waveform to rise and the phase of ensuing vibration.
That is, in case the drive voltage waveform rises at a time when the speed of ensuing vibration is increasing toward the pressure chamber, the ink droplet grows in volume and the flying speed increases. When the drive voltage waveform rises at a time when the speed of ensuing vibration is decreasing toward the pressure chamber, on the other hand, volume of the ink droplet and the flying speed decrease.
As a consequence, a slightest variation in the pulse width of the drive voltage waveform results in significant variations in the volume of the ink droplet and the flying speed.
Also because thickness of the piezoelectric element and the conditions of bonding onto the oscillator plate vary among the plurality of piezoelectric elements disposed on the piezoelectric ink jet head, there are differences in the natural period of vibration among the drive sections. As a result, there occur variations in the volume of the ink droplet and the flying speed among the nozzles, even when the pulse width of the drive voltage waveform is maintained constant.
These problems arise similarly in the Push-pull drive method.
In order to suppress the ensuing vibration of the drive section, Japanese Unexamined Patent Publication JP-H05-318731-A1 (1993) proposes such a Pull-push drive method as described below. Time constant of decreasing voltage is set at 0.9 times the natural period of vibration of the drive section or longer when the drive voltage waveform falls, namely when removing the drive voltage Vp from VH to zero at time t1 in FIG. 17, and time constant of increasing voltage is set in a range from 0.9 to 1.2 times the natural period of vibration when the drive voltage waveform rises, namely when applying the drive voltage Vp from zero to VH at time t2 in FIG. 17.
It is true that ensuing vibration of the drive section can be suppressed by increasing the time constant of rise/fall. However, increasing the time constant of rise/fall leads to another problem that the flying speed of ink droplet decreases.
Japanese Patent Unexamined Publication JP-H05-318731-A1 employs a piezoelectric element longitudinal vibration mode that is formed in the shape of a thick plate or a rod having a predetermined cross section that expands in the direction of plate thickness or longitudinal direction of the rod when subjected to a drive voltage.
Since a piezoelectric element of longitudinal vibration mode has smaller natural period of vibration of the drive section compared to one of transverse vibration mode, the flying speed of ink droplet does not decrease significantly even when the time constant of rise/fall of drive voltage waveform is made as long as similar to the natural period of vibration of the drive section.
However, the piezoelectric element 9 of transverse vibration mode shown in FIGS. 2 and 3 has larger natural period of vibration of the drive section than that of longitudinal vibration mode. As a result, the flying speed of ink droplet decreases significantly when the time constant of rise/fall of drive voltage waveform is made as long as similar to the natural period of vibration of the drive section.